ENH: add chunksize argument to read_sql (GH2908)

Closes #2908 

Should probably add this also to `read_sql_table`. And try to reduce some repetitions (`_wrap_result` can maybe be a top-level (in the file) helper function).
I think your class would be simpler (and u could inherit methods)
and bundle wrap_results - really just preference - up 2 u